Conventionally, sensorless vector control of a synchronous motor employs, as inputs, a difference between a stator current converted into a γ-δ coordinate system set on poles of a rotor and a current estimated most recently and a voltage instruction converted into the γ-δ coordinate system, thereby estimating an electric current and induced voltage of the γ-δ coordinate system and the speed of the rotor.
Through use of a γ-axis induced voltage and an angular speed of the rotor, which are estimated by this method, an angle of error between a d-q coordinate and the γ-δ coordinate set on permanent magnets of the rotor is estimated, whereby the position of the rotor is modified.
Vector control of the motor is performed through use of the angular speed and information about the position of the magnetic axis, which are estimated by the above method.
However, the background art technology encounters a problem. Namely, as a synchronous motor rotates at low speed, a voltage induced by the synchronous motor decreases, thereby deteriorating the accuracy of estimation of the magnetic axis. If vector control of the synchronous motor is performed within a low-speed range, the magnetic axis is lost. Accordingly, the synchronous motor can no longer be controlled.
When large torque is exerted on the synchronous motor within a low-speed range, the angle of load becomes excessively wide, and a difference in angle between a control axis and the magnetic axis of the synchronous motor becomes greater. As a result, a smooth shift toward vector control for controlling the synchronous motor while the control axis is aligned with the magnetic axis fails, thereby causing a problem of the synchronous motor no longer being under control.
Accordingly, the present invention is aimed at providing a superior method of estimating the speed of a synchronous motor, which method enables accurate specification of a magnetic axis even within a low-speed range. Particularly, a first object of the present invention is to provide a method of controlling the speed of a synchronous motor, which method enables realization of a favorable shift toward vector control by means of aligning a control axis with a magnetic axis in the event of great torque being exerted on the synchronous motor within a low-speed range.
Noting that a converter for vector control purpose can accurately control the magnitude, frequency, and phase of an output current, there has been proposed a method comprising the steps of supplying a predetermined current to a motor, measuring a current constant of the induced motor with high accuracy on the basis of a motor voltage induced by the current, and setting, on the basis of a result of measurement, a control-operation constant of an induced motor control system (Japanese Patent Application Laid-Open No. 183953/1985).
However, a method of identifying the constant of a synchronous motor has not yet been proposed for a controller for driving the motor. The control constant of the motor has hitherto been set on the basis of a design constant of the motor. The control constant of each motor to be used must be changed, thus involving an element of inconvenience. The disparity between the design value and a real value induces occurrence of an error in control operation, thereby deteriorating the operating performance of the motor. If the control constant of a motor is manually measured through use of a measuring instrument, there arises a problem of consuming time and deteriorating the accuracy of the constant of the motor.
A second object of the present invention is to provide a method of identifying an induced voltage constant and a d-axis inductance of a motor with high accuracy and at high speed.